"Article in series Rewriting the Social Contract: Barriers to Middle-Class Pay; longstanding wage advantage that manufacturing workers enjoy over their counterparts in services or construction is shrinking fast as older workers depart, at Caterpillar and at other companies; United Auto Workers union at Caterpillar reluctantly accepted two-tier contract that provides significantly lower wages and benefits for newly hired employees; new second tier is as much as $20 per hour below cost for older workers; trade-off is promise of manufacturing revival at long last in old Rust Belt, as new hires come aboard at much lower labor costs; even healthy and highly profitable companies like Caterpillar are engaging in practice of two-tier wage structure; longstanding presumption that factory workers at successful companies can achieve secure, relatively prosperous middle-class life for themselves and their families is evaporating; Caterpillar chief executive Jim Owens says lowering wages has allowed company to add jobs; graph; photos (L)"